Arrangement for transmission and reception of electro-magnetic radiation

ABSTRACT

Arrangement for transmission and reception of electromagnetic radiation. Aransmitting and receiving part (1) transmits and receives the electromagnetic radiation, and a reflecting part (2) reflects the transmitted electromagnetic radiation back to the transmitting and receiving part while passing through a medium which transmits radiation. The transmitting and receiving part has an electromagnetic radiation source (5) and a concave main reflector (3) which is common to transmission and reception. The transmitting and receiving part has a second reflector (6), which is positioned between the radiation source (5) and the main reflector (3), so that this second reflector shades a part of the main reflector reflecting area (4) for the radiation transmitted by the radiation source towards the main reflector. This forms a shadow area (17) on the main reflector reflecting area for transmitted radiation. This second reflector receives a part of the reflected radiation from the main reflector and reflects this back towards a detector for reception of the radiation. The shadow area for transmitted radiation also has a reflecting receiving area for the radiation which is incident upon the main reflector and reflected back towards the detector by the second reflector. The remaining part of the main reflector reflecting area, which is not shadow area for transmitted radiation, forms a transmitting area for the radiation which is transmitted by the radiation source and reflected in the main reflector.

BACKGROUND OF THE INVENTION

The present invention relates to an arrangement for transmission andreception of electromagnetic radiation, consisting of a transmitting andreceiving part for transmission and reception of the electromagneticradiation, and a reflecting part, arranged so as to reflect thetransmitted electromagnetic radiation back to the transmitting andreceiving part, while passing through a medium which transmitsradiation. The transmitting and receiving part includes anelectromagnetic radiation source and a concave main reflector which iscommon to transmission and reception.

Transmission and reception of electromagnetic radiation is of interestin a number of applications, for example in measurement of airpollution, distance measurement etc. In this connection, use is made ofvisible or invisible light, for example, which is transmitted over ameasuring distance, after which the light is collected and analyzed. Inorder to be able to transmit light over great distances without toogreat losses in intensity, use is made of telescopes. In thisconnection, a light source is positioned close to the focus of aparabolic or spherical mirror. After reflection in the mirror, light inthe form of a parallel luminous beam is transmitted. After transmissionthrough the atmosphere, a part of the luminous beam is collected with areceiving telescope constructed in similar manner and is focussed into aspectrometer, either by direct connection or via an optical fibre. Inthe case of direct connection, use is made, for example, of a so-calledNewton arrangement with a mirror arranged at an angle in front of thereceiving mirror.

An alternative to positioning transmitter and receiver at either end ofthe measuring distance is to transmit the radiation out and back overthe measuring distance via a reflector. In this connection, all thecomponents in the system must be accurately aligned, since transmitterand receiver are normally positioned next to one another, so that theradiation which is reflected back must be directed in a differentdirection from the transmitted radiation.

A suitable type of reflector for positioning at the far end of themeasuring distance would be a so-called retroreflector, which is anoptical component in which a number of reflecting areas are arranged insuch a manner that the light which strikes the same is returned in theopposite direction, even in the case of incident light which deviates toa certain extent from the direction of the normal. By virtue of the factthat a retroreflector usually has a relatively high angular tolerance,the requirement for alignment and stability is relatively low. However,use of retroreflector in the known arrangement described above, withtransmitter and receiver positioned side by side, would still occasionproblems. The maximum intensity of the reflected light would in fact goback into the transmitting telescope and only a small part would reachthe receiver.

SUMMARY OF THE INVENTION

The aim of the present invention is to bring about an arrangement fortransmission and reception of electromagnetic radiation, in whichtransmitter and receiver are positioned in the same place and in whichthe arrangement is easy to align.

Said aim is achieved by means of an arrangement according to the presentinvention, which is characterized in that the transmitting and receivingpart has a second reflector, which is positioned between the radiationsource and the main reflector, so that this second reflector on the onehand shades a part of the main reflector reflecting area for theradiation transmitted by the radiation source towards the mainreflector, forming a shadow area on the main reflector reflecting areafor transmitted radiation, and on the other hand receives a part of thereflected radiation from the main reflector and reflects this backtowards a detector for reception of the radiation, which shadow area fortransmitted radiation also comprises a reflecting receiving area for theradiation which is incident upon the main reflector and reflected backtowards the detector by the second reflector, the remaining part of themain reflector reflecting area, which is not shadow area for transmittedradiation, forming a transmitting area for transmitted radiation,forming a transmitting area for the radiation which is transmitted bythe radiation source and reflected in the main reflector.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail below using an exemplaryembodiment with reference to the attached drawings, in which

FIG. 1 shows schematically an arrangement according to the inventionwith the ray path for transmitted radiation drawn in,

FIG. 2 shows schematically an arrangement in which the ray path for theincident radiation is drawn in,

FIG. 3 indicates the main reflector reflecting area which is used upontransmission of the radiation and

FIG. 4 shows the main reflector reflecting area which is used uponreception of incident radiation.

DETAILED DESCRIPTION

The example shown envisages an arrangement for measuring air pollution,with which light is transmitted over a measuring distance of, forexample, 100 m to 10 Km, after which the light is collected and analyzedby means of a spectrometer. In this connection, an absorption spectrumof a given gas is selected between a transmitter and a receiver, thespectrometer measuring the intensity of the received light as a functionof the wavelength within a given wavelength band. According to theexample shown, electromagnetic radiation of the light type is used,which can lie within the visible or invisible wavelength range. To thisend, the arrangement according to the invention has a combinedtransmitting and receiving part 1 and a reflecting part 2, whichconsists of a separate unit situated at a distance from the transmittingand receiving part with, in between, the gaseous medium which is toanalyzed. The transmitting and receiving part 1 is made up of a mainreflector 3 in the form of a concave mirror, which is expedientlydesigned with a parabolic or spherical mirror surface 4. A light source5 in the form of a lamp is positioned on the optical axis 13 of thearrangement, close to the focus of the concave mirror 3. Between thelight source 5 and the concave mirror 3, a second reflector 6 ispositioned, which forms part of the transmitting and receiving part andthe reflecting area 7 of which is turned towards the reflecting area 4of the concave mirror 3. According to the example shown, this secondreflector is designed as a plane mirror. Furthermore, both the concavemirror 3 and the plane mirror 6 are advantageously designed withcircular shape, that is to say forming with their periphery a closedcircular curve. This circular curve of the plane mirror has a radius ofcurvature which is less than the radius of curvature of the periphery ofthe concave mirror 3, that is to say the plane mirror 6 covers an areawhich is considerably smaller than the area of the concave mirror 3.

An example of the dimensional relationships, in the case of a distanceto the reflecting part 2 of approximately 1 km, is as follows: thedistance along the optical axis 13 between the radiation source 5 andthe concave mirror 3=1.2 m, the distance between the two mirrors 3 and6=0.9 m and thus the distance between the light source 5 and the planemirror 6=0.3 m. The concave mirror 3 has a diameter with regard to itsperiphery of, for example, 0.3 m, whereas the plane mirror 6 should inthis connection have a diameter of approximately 0.05 m.

The reflecting part 2 has a diameter of, for example, approximately 0.15m and is advantageously circular accordingly. In principle, thereflecting part 2 can consist of a plane mirror but advantageouslyconsists of a retroreflector, which reflects the incident light in theopposite direction parallel to the incident light within a given angularrange for the angle of incidence. By these means, the exact alignment ofthe reflecting part 2 becomes non-critical.

In FIG. 1, for the sake of clarity, only the ray path in the case oftransmission of the radiation and only the components which influencethis are shown. In this connection, it has been indicated above that thelight source 5 is positioned close to the focus, which in practice meansthat it is not necessarily positioned exactly at the focus, resulting inthe rays reflected by the concave mirror 3 not being completelyparallel, owing to the fact that the reflecting part is not positionedat infinite distance, for which reason the transmitted ray beam has rays8, 9 which are slightly convergent. These deviations are, however, verysmall, for which reason it is possible to speak of the light sourcebeing positioned in the general area of the focus and of the transmittedrays from the concave mirror being essentially parallel.

FIG. 2 shows in a corresponding manner only the ray path for the rays 9,11 which are reflected from the reflecting part 2 and incident upon thetransmitting and receiving part 1. A collecting element for the raybeam, which is situated on the optical axis 13 at a point 12 between theconcave mirror 3 and the plane mirror 6, belongs to the receivingcomponents in the transmitting and receiving part. This collectingelement can consist of a light guide for guiding the collected light toa spectrometer for spectral analysis of the received ray beam.

By means of the arrangement according to the invention described above,with the smaller mirror 6 positioned between the light source 5 and thelarger, concave mirror 3 and turned towards the latter, and with a sizeof the smaller mirror which is selected in relation to on the one handthe positioning of the smaller mirror and on the other hand the size ofthe larger, concave mirror, it is brought about on the one hand that thesmaller mirror shades the larger mirror from the rays which aretransmitted from the light source and directed towards the larger mirrorand on the other hand that, by means of the smaller mirror, only a partof the essentially parallel rays, which are incident upon the largermirror and reflected back, is received. This is seen in both FIG. 1 andFIG. 2, and is clarified schematically in FIGS. 3 and 4, which show thedifferent zones into which the surface of the concave mirror 3 can bedivided by means of the effect of the smaller mirror 6.

The mirror surface 4 of the concave mirror can thus be divided intothree zones, namely a first zone 14, which is annular in the exampleshown, constitutes a transmitting area and is hatched in FIG. 3, asecond zone 15, which is likewise annular in the example shown,constitutes a receiving area and is hatched in FIG. 4, and a third,circular zone 16, which forms a shadow area for incident rays and is notused. The shape of these zones is determined by the shape and mutualpositioning of the two mirrors 3, 6 and in the example shown the mirrorshave a circular outer contour and are positioned symmetrically inrelation to the optical axis 13. According to the invention, thereceiving area 15 lies within an area which is at the same time a shadowarea 17 for the radiation transmitted by the light source 5 towards theconcave mirror 3.

Briefly, the arrangement according to the invention functions in thefollowing manner, the transmitted radiation, which is shown in FIG. 1,being described first. A ray beam is transmitted from the light source5, preferably in a direction which is essentially towards the concavemirror 3. The latter is struck by the ray beam in the first zone 14,that is to say the transmitting area, since a part of the mirror surface4 is shaded by the smaller, plane mirror 6 in such a manner that theshadow area 17 for the transmitted ray beam is formed. By virtue of thefact that the light source 5 is situated in the area of the focus of theconcave mirror 3, the annular ray beam is reflected in the concavemirror in essentially parallel rays, which are propagated through thegaseous medium which is to be analyzed. As a result of a diffraction,aberration, reproduction effects and, where appropriate, parallel shiftin the reflecting part 2, the transmitted light beam with annularcross-section becomes "fused" and is returned, after reflection in thereflecting part 2, as a collected light beam. The ray path described,from the light source 5 and to the reflecting part 2 after passagethrough the gaseous medium, can thus be seen from FIG. 1. Parallel shiftoccurs in the reflecting part 2 in the event of use of a suitablereflector, such as one of the types of retroreflector available on themarket. A suitable retroreflector is, for example, made by PrecisionLapping Optical Co. U.S.A. By means of parallel shift in the reflectingpart, the rays undergo, upon reflection, parallel shift through adistance which is determined by the construction of the reflector, whichthus contributes to the fact that the transmitted annular ray beambecomes "fused". The extent of the parallel shift is adapted to theradial dimension of the receiving area 15 and the transmitting area 14.

The ray path after reflection in the reflecting part 2 is to bedescribed now with reference to FIG. 2. The reflected ray beam 10, 11accordingly passes back through the gaseous medium as essentiallyparallel rays and again strikes the mirror surface 4 of the concavemirror 3, with the exception of that part of the incident ray beam whichstrikes the plane mirror 6 and is thus obscured by the latter, as aresult of which the shadow area 16 is formed for incident rays. However,not all the ray beam which is incident upon the concave mirror 3 andreflected by the latter strikes the plane mirror 6 on its mirror surface7. This depends upon the relationship between the size of the twomirrors and the positioning of the smaller mirror 6 along the distancebetween the light source 5 and the concave mirror 3. By these means, thereceiving area 15 is created, which thus falls within the shadow area 17for the transmitted ray beam and thus forms a common circular boundaryline 18 between shadow area 17 and the transmitting area 14 fortransmitted rays and between the receiving area 15 for incident rays andsurrounding area 19 of the mirror surface 4 of the concave mirror 3. Thearea 19 brings about reflection of the rays to the areas outside theouter contour of the plane mirror 6. The fact that the ray beam, whichis incident upon the concave mirror 3 and reflected by the reflectingpart 2, arrives to a large extent within the boundary line 18, that isto say within the receiving area 15, depends, as mentioned above, on theone hand on imperfections in the optical components and refractionphenomena in the transmission medium and on the other hand also, in thechosen example, on parallel shift in the reflecting part, by which meansthe light rays can be "shifted in" to the receiving area 15 with aselected parallel shift. The ray beam, which is incident upon the mirrorsurface 7 of the plane mirror 6, is reflected in the mirror surface 7 insuch a manner that the ray beam converges towards the point 12, that isto say towards the collecting element, to be conveyed onward via, forexample, the light guide or mirrors arranged at an angle towards a placeoutside the incident ray beam to a spectrometer for special analysis.

With the arrangement described above, transmitting and receiving partcan thus be arranged in the same place without having to be positionedside by side, which leads to a number of advantages, such as:

only one component must stand in a stable manner and be alignedaccurately;

all complicated components are in one and the same place;

only one place requires a power supply;

measurement over several distances can be made more economical;

the same light source as is used in measurement is also available forcalibration for compensation of instrument factors;

the measuring distance can be halved while retaining sensitivity.

The invention is not limited to the exemplary embodiment described aboveand shown in the drawings, but can be modified within the scope of thefollowing claims. It is not, for example, in itself necessary that theplane mirror 6 is positioned symmetrically on the optical axis 13, butit can even be positioned in such a manner that it only partiallyprojects into and shades the ray beam. The same effect occurs in fact,irrespective of the laterally displaced position of the mirror inrelation to the optical axis, namely that the receiving area 15 fallswithin the shadow area which occurs for the transmitted ray beam. Theradiation source 5 also can be positioned outside the optical axis 13.Furthermore, it is not necessary that the mirrors are designed with acircular outer contour, but they can have another shape. The smallermirror 6 does not necessarily need to be plane, but can also be concaveor convex. Other types of telescope principle can be used, for examplethe Cassegrain type, in which case the point 12 is behind the concavemirror 3. In the latter case, an opening is arranged in the concavemirror for the passage of the rays which are reflected back from thesmaller mirror.

In the example shown, an arrangement for measuring air pollution bymeans of spectral analysis is considered. The construction and thefunctioning of the arrangements after the collection at point 12 can beof per se conventional type, for which reason these should require nomore detailed description. Exactly the same arrangement, with theexception of the part which concerns the spectrometer, can be used fordistance measurement, in which case the distance is quite simplydetermined starting from measurement of the time lag between transmittedray beam and received ray beam, which is indicated, for example, inpulses, and with knowledge of the velocity of propagation of theradiation. Alternatively, the distance can be determined by means ofmeasurements of phase shift of monochrome transmitted light. Thearrangement can also be used for completely different purposes and canin this connection be used in conjunction with electromagnetic radiationof another type, for example, microwave radiation or radio waves. It iseven conceivable that radiation within completely different wavelengthranges is possible, such as sound waves, for example ultrasound. Themedium in between, through which the ray beam is transmitted, can alsobe a liquid or solid medium.

The collecting element 12 can be situated outside the light beam, whichis incident upon the concave mirror 3, in which case the smaller mirror6 is arranged at an angle of e.g. 45° for a 90° deflection of the raybeam, which is incident upon the smaller mirror, to the collectingelement which in this connection can be constituted by the light inputto the spectrometer.

The reflecting part 2 can also consist of a simple plane mirror. Use ofa retroreflector with parallel shift is thus not a necessaryprerequisite for the invention, but rather an advantageous embodiment.

In the claims, reference is made to the positioning of the detector. Thecollecting element, which can accordingly convey the collected lightbeam onwards to another place, is in this connection equated with theterm detector.

As in the case in most optical systems, the ray path can be reversed,while retaining the basic function. In this connection, however, thelight source 5 and the collecting element or the detector 12 must changeplaces. The light source is thus positioned between the main reflector 3and the second reflector, while the collecting element 12 or thedetector is positioned beyond the reflector 7, at the focus of the mainreflector in the example shown. The light source, in the example shown,is positioned in such a manner that the sum of the distance between themain reflector 3 and the second reflector 6 and between the latter andthe light source is equal to the focal distance of the main reflector.Furthermore, the size and position of the reflector are selected in sucha manner that the radiation transmitted by the light source, afterreflection in the reflector 6, illuminates a part of the main reflector,but not all. The part of the main reflector illuminated in this mannerconstitutes a transmitting area, while the remaining part of the mainreflector constitutes a receiving area for the radiation which returnsfrom the retroreflector 2. The part of the returning radiation whichfalls within the receiving area is focussed to the collecting element12. The reflector 6 also has the characteristic of shading thecollecting element from direct or reflected (but not via theretroreflector) radiation from the light source.

I claim:
 1. An apparatus for the transmission and reception ofelectromagnetic radiation comprising:a.) a concave reflector having afirst central focal axis and an outer defining edge with a firstcircular portion centered about said central focal axis for receivingelectromagnetic radiation impressed thereon and a second circularportion centered about said central focal axis and extending from theoutside edge of said first circular portion to the edge of saidreflector for transmitting electromagnetic radiation impressed thereon;b.) a first reflector having a second central focal axis, said secondcentral focal axis of said first reflector corresponding with said firstcentral focal axis of said concave reflector said first and second focalaxes creating a common focal axis, said first reflector being spaced afirst predetermined distance from said concave reflector along saidcommon central focal axis, said first reflector reflecting transmittedelectromagnetic radiation from said second circular portion of saidconcave reflector passing through a medium which permits the passage ofelectromagnetic radiation therethrough, back to said first circularportion of said concave reflector through said medium as reflectedradiation; c.) an electromagnetic radiation source positioned on saidcommon focal axis of said concave and said first reflectors; d.) adetector for receiving electromagnetic radiation reflected from saidfirst circular portion of said concave reflector, said detector beingpositioned on said common focal axis between said concave reflector andsaid electromagnetic radiation source; and e.) a second reflectorpositioned between said electromagnetic radiation source and saidconcave reflector on said common focal axis to permit said secondreflector to shield said first circular portion of said concavereflector when electromagnetic radiation is applied to said secondcircular portion of said concave reflector by said electromagneticradiation source so as not to receive such electromagnetic radiation andto shield said second circular portion of said concave reflector whenelectromagnetic radiation is applied to said first circular portion ofsaid concave reflector from electromagnetic radiation reflected fromsaid first reflector, said second reflector reflects a portion of thereflected radiation from said first circular portion of said concavereflector to said detector.
 2. An apparatus as defined in claim 1,whereina.) said concave reflector has a focal length and an area offocus and said second reflector is separated from said concave reflectoralong said common central focal axis by a first distance and said secondreflector is spaced from said electromagnetic radiation source alongsaid common central focal axis by a second distance, and the sum of saidfirst and said second distances equals the focal length of said concavereflector resulting in a substantially parallel beam of electromagneticradiation being transmitted from said second circular portion of saidconcave reflector; b.) said first reflector receives said parallel beamof radiation and reflects said parallel beam of radiation in asubstantially parallel form to said first circular portion of saidconcave reflector which in turn reflects said beam towards the area offocus of said concave reflector; c.) said detector separated from saidsecond reflector along said common focal axis by a third distance, thesum of said first distance and said third distance being equal to thefocal length of said concave reflector.
 3. An apparatus as defined inclaim 2 wherein said second reflector is positioned along said commonfocal axis and is of a diameter, with relation to the diameter of theconcave reflector, that said first and second circular portions of saidconcave reflector are of essentially the same size.
 4. An apparatus forthe transmission and reception of electromagnetic radiationcomprising:a.) an electromagnetic radiation source; b.) a transmittingand receiving device for the transmission and reception ofelectromagnetic radiation initially emanating from said radiationsource; c.) said transmitting and receiving device comprising: a concavereflector having a first central focal axis, a focal point and an outerdefining edge, with a first circular portion centered about said firstcentral focal axis for transmitting electromagnetic radiation impressedthereon and a second circular portion centered about said first centralfocal axis and extending from the outside edge of said first circularportion to the edge of said reflector for receiving electromagneticradiation impressed thereon; d.) a first reflector arranged to reflectthe transmitted electromagnetic radiation from said second circularportion of said concave reflector passing through a medium which permitsthe passage of electromagnetic radiation therethrough back to said firstcircular portion of said concave reflector through said medium asreflected radiation; e.) a detector for detecting reflected radiation;f.) a second reflector positioned along a second central focal axiscommon with said first central focal axis of said concave reflector,said first and second focal axes forming a common focal axis andpositioned along said common focal axis at a position between thesurface of said concave mirror and the focal point of said concavereflector; g.) said second reflector having a reflecting surface facingthe reflecting surface of said concave reflector; h.) said radiationsource being positioned along said common focal axis of said concavereflector and said second reflector; i.) said second reflector beingarranged to direct radiation from said radiation source to said firstcircular portion of said concave reflector while preventing suchradiation from falling upon said second circular portion of said concavereflector or directly upon said detector from said radiation source;said first circular portion of said concave reflector arranged totransmit radiation towards said first reflector; j.) said secondcircular portion of said concave reflector arranged to receive radiationreflected from said first reflector and reflects such radiation to saiddetector.
 5. An apparatus according to claim 4, wherein said concavereflector has a focal length and an area of focus, and said secondreflector is spaced from said concave reflector along their common focalaxes by a first distance and said second reflector is spaced from saidelectromagnetic radiation source along their common focal axis by asecond distance, the sum of said first and second distances beingequivalent to the focal length of said concave reflector resulting in asubstantially parallel beam of electromagnetic radiation beingtransmitted from said first circular portion of said concave reflector;said second circular portion of said concave reflector is arranged toreflect a beam of electromagnetic radiation impinging upon it in asubstantially parallel beam in a direction towards said area of focus ofsaid concave reflector; said second reflector spaced along said commonfocal axis from said concave reflector by a first distance and saiddetector is spaced from said second reflector by a third distance, thesum of said first and third distances being equal to the focal length ofsaid concave reflector.
 6. An apparatus as defined in claims 2 or 5wherein said first reflector is retroreflective and reflects saidreflected radiation from said first reflector substantially in parallelto said transmitted beam of radiation within a prescribed angular rangecorresponding to the angle of incidence of the transmitted beam ofradiation upon said first reflector.
 7. An apparatus as defined in claim6 wherein said concave reflector and said second reflector each have anessentially circular configuration and that said second reflector ispositioned along said common focal axis and separated from saidelectromagnetic radiation source by a distance approximately equal to1/4 of the distance between said source of electromagnetic radiation andsaid concave reflector.
 8. An apparatus as defined in claims 1 or 4,wherein said radiation source comprises a light source and said concavereflector and said second reflector are each mirrors.
 9. An apparatus asdefined in claims 1 or 4 wherein said first reflector is arranged toeffect a parallel shift of the beam of radiation upon reflection fromsaid first reflector in an amount in dependence upon the radialdimension of said second circular portion of said concave reflector withrespect to the radial dimension of said first circular portion of saidconcave reflector.
 10. An apparatus according to claim 1 or 4 whereinsaid second reflector consists of a plane mirror positioned on saidfirst focal axis of said concave reflector and with its plane faceperpendicular to said focal axis of said concave reflector.
 11. Anapparatus as defined in claims 2 or 5, wherein said first circularportion of said concave reflector is arranged to reflect electromagneticradiation impinging thereon with a parallel shift depending upon theradial dimension of said first circular portion of said concavereflector as compared to the radial dimension of said second circularportion of said concave reflector.
 12. An apparatus as defined in claim11, wherein said second reflector is positioned symmetrically on saidfirst central focal axis of said concave reflector and has a diameterwhich is between 0.15 and 0.20 times the diameter of said concavereflector.
 13. A method of employing the apparatus of claims 1 or 4,said method comprising positioning said apparatus in a medium throughwhich electromagnetic radiation is passed, and analyzing the reflectedradiation received by said detector to determine the composition of themedium through which said electromagnetic radiation was passed.
 14. Themethod according to claim 13, in which said transmitted radiation iscollected in said detector which forms part of a spectrometer forspectral analysis.